The present invention relates to the field of gas turbine control systems and, in particular, to water-injection systems that saturate and/or supersaturate with water the air stream entering the gas turbine.
The saturation and supersaturation of air with water at the inlet of a gas turbine generally increases turbine power output. The injection of water into the inlet air is often done when operating gas turbines in conditions of high ambient-air dry-bulb temperature. Water will generally evaporate as it is injected into the compressor inlet air and before the air/water mixture enters the compressor inlet, provided that the mixture is no more than saturated with water as it enters the compressor.
Existing water-injection systems saturate or supersaturate the compressor inlet air to augment the gas turbine power output. Mechanical water-injection systems are available that cool the inlet air supplied to a gas turbine compressor by injecting water through an array of manifolds and atomizing nozzles. Water evaporation desirably cools the air entering the inlet guide vanes (IGVs) of the compressor. Generally, the lowest working fluid inlet temperature is achieved by saturating the inlet air with water. Thus, power output can be increased in a gas turbine with a water-injection system.
Supersaturation of the air/water mixture at the compressor inlet provides water in the inlet air that is not evaporated as it enters the compressor. Supersaturation performs at least two beneficial functions useful for increasing gas turbine power output including:
1. Injected water (which is not evaporated as it enters the compressor) does eventually evaporate as the working fluid (air/water mixture) flows through the compressor. Inter-compressor evaporation cools the working fluid passing through the compressor. Cooling of the working fluid within the compressor (intercooling) reduces the power required for its compression. The net gas turbine power output is the total turbine power output minus the compressor power input requirement. Any reduction in the power required by the compressor results in a net increase in gas turbine power output. This net power output gain is substantially equal to the reduction in compression power input.
2. The mass flow of the working fluid is increased by supersaturation. The compressor provides a constant volume of working fluid to the combustor of the gas turbine. The mass of the working fluid can be increased by supersaturating the air/water mixture entering the compressor. The specific volume of the liquid water mist is approximately {fraction (1/800)}th of the specific volume of the air. Inversely, the mass flow of a supersaturated water mist is greater than that of a saturated air/water working fluid mixture. Turbine power output is directly proportional to the mass flow of working fluid. Due to its high mass, the added water in a supersaturated working fluid increases gas turbine power output. Accordingly, supersaturating the working fluid with water increases the mass flow of that fluid and the power output of the turbine.
The power output of a gas turbine can be increased when operating at high ambient-air dry-bulb temperature by injecting finely atomized water into the air flowing into the intake duct of the compressor, so that the compressor inlet air is supersaturated when entering the compressor. Conventional supersaturation water-injection systems might consist of a single set of manifolds and nozzles in the duct, or might be divided into two injection sections of manifolds and nozzles. Gas turbine power augmentation can be maximized by dividing the injection system into two sections, the upstream section which saturates the air with water and a second downstream section located at or near the compressor inlet that supersaturates the air.
An atomized water spray-type saturation system (fogger) with controls is described in U.S. Pat. No. 5,463,873. Prior systems for supersaturating the air supplied to a compressor inlet of a gas turbine have generally consisted of a single grid of atomized spray nozzles located in the air intake duct. Systems typical of these are described in U.S. Pat. Nos. 5,867,977 and 5,930,990. European Patent No. EP 0 781 903 A2, entitled xe2x80x9cGas Turbine with Water Injectionxe2x80x9d describes systems for supersaturating air supplied to the compressor inlet of a gas turbine, including a divided saturation and supersaturation system. A divided water-injection/cooling system divides the water-injection system into two sets of manifolds and nozzles. A first injection system of manifolds and nozzles provides sufficient atomized water to xe2x80x9csaturatexe2x80x9d the inlet air, and sufficient residence time of the water in the air to enable complete evaporation of the water in the air before it enters the IGVs of the compressor. The water-saturated air from the first injection system enters a second injection system with its own set of manifolds and nozzles to inject additional water in order to supersaturate the air.
A potential danger of supersaturating the air/water mixture entering the compressor is that water droplets can damage the compressor. Droplets can agglomerate from the fine water mist in the fluid or from excess water mist that is injected into a supersaturated air/water mixture. Large water droplets in the mist can impact and corrode compressor blades and stators. Blade erosion can occur due to large water droplets in the working fluid of a compressor. A control system is needed to operate water-injection systems for gas turbines. The control system is needed to maximize power augmentation due to water injection, and to avoid the compressor blade erosion and other harmful effects of excessive water injection.
Applicants have invented novel control systems for saturation and saturation/supersaturation water-injection systems for gas turbines. The control systems regulate the quantity of water injected into the inlet air supplied to the compressor by the saturation and/or supersaturation sections of a water-injection system. The control systems optimize power augmentation and limit water injection to comply with certain gas turbine limitations. These control systems also regulate the injection of water to maximize the power increase and minimize the potential for compressor-blade erosion.
Specifically, the control systems start, stop and modulate/regulate the quantity of water injected into the compressor air. These systems maximize power output of the gas turbine, minimize erosion of the compressor blades, and limit operation of the power-augmentation system to suitable and advantageous conditions. When the water-injection grids (e.g., manifolds and atomizing nozzles) are divided, water injection to each grid is controlled so that the air is at or near saturation when discharging from the saturating grid, and then modulates the water flow to the supersaturating section, within limits as indicated by air flow and other gas turbine parameters.
Individually controlling the water injected by each section of a divided injection system provides several advantages including:
(i) Ensuring that the injected air fully saturates the air/water mixture at the compressor inlet. Saturating the air using a first injection grid enables the temperature of the air/water mixture to be reduced to near or at the wet-bulb temperature for the air/water mixture by the first grid. The temperature is reduced due to evaporation of the water added by the first system.
(ii) Increasing the power output of the gas turbines by reducing as the temperature of the working fluid (air/water mixture) entering and passing through the compressor. The power output increases because of: (1) an increase in cycle mass flow of the working fluid through the gas turbine caused by an increase in the density of that fluid, and (2) an increase in the cycle temperature ratio (ratio of firing temperature to compressor inlet temperature) due to a decrease in compressor inlet temperature.
Separating the water mist injection systems for saturating and supersaturating sections of the integrated system and modulating the water flow to each individually enables the system to accommodate widely varying atmospheric humidity conditions, while maintaining air/water mixture composition to optimize gas turbine power output while simultaneously maintaining the operation of the supersaturating system within satisfactory limits to prevent erosion of compressor blades. If the ambient-air humidity is low, a significant amount of water can be injected into the air in the saturating section, while the maximum quantity of water can still be safely injected in the supersaturating section.
If the ambient air is already saturated, no water can be evaporated in the saturating system. In such a saturated ambient-air condition, the control system does not inject water into the saturating section of the injection system. Although water should not be injected to the saturation system when the ambient air is saturated with water, water can still be injected by the supersaturating section. Thus, water injection can be performed to augment gas turbine power output, even in high humidity conditions.
The controls for the water-injection systems avoid injecting water, such that ice will form in the compressor. The dry-bulb temperature for the water-saturated air entering the gas turbine compressor should be maintained above the temperature where there is potential for freezing of water on the compressor IGVs or first-stage rotor blades. This temperature must be maintained sufficiently above the water freezing point to avoid freezing the water in the air/water mixture as it is accelerated through the inlet volute and compressor IGVs. If ice forms on the compressor IGVs or rotating blades, air flow is reduced, which reduces power output. Also, if ice accumulates and subsequently breaks off the IGVs or first-stage rotor blade, the ice can cause significant mechanical damage to downstream stages. The controls for the water-injection system should exclude operation of the saturation and supersaturation system during conditions when freezing of water in the compressor may occur.
Saturation of the gas turbine inlet air will generally increase the thermal efficiency of the gas turbine (or a combined steam and gas turbine power generation system) within which the gas turbine is incorporated when the gas turbine is at or near maximum output. When the operator sets the power output at a load lower than maximum power output, the thermal efficiency is reduced by operating the saturating system because the gas turbine will operate at a point further from its design point, where the design point is the best efficiency point. Supersaturation of the gas turbine inlet air will, at lower loads, similarly decrease the thermal efficiency of the gas turbine or combined cycle system. Therefore, the control system may exclude operation of the saturation system and/or supersaturation systems, except when the gas turbine is operating near or at its maximum output.
An embodiment of the disclosed water-injection control system is for a water supersaturation system installed in a gas turbine air intake system for augmenting gas turbine power output during operation at high ambient-air temperature, while maintaining the water content in the intake air within limits acceptable to the gas turbine. The mechanical components of the injection system include an atomized water spray-type supersaturation system consisting of multiple manifolds and atomizing nozzles installed in the gas turbine air intake duct or in the gas turbine compressor inlet hood and an auxiliary system for delivering a controlled flow of water to each manifold that consists of a water storage tank, water pump, piping control valves, and flow sensors. The water injection control system starts, stops, modulates, and limits water supplied to the supersaturating system based on the ambient-air humidity or dew-point temperature, ambient-air dry-bulb temperature, air flow at the compressor inlet, and gas turbine parameters indicating operation at or near maximum output.
Another embodiment of the water injection control system is for an atomized-water spray-type evaporative cooler installed in a gas turbine air-intake system for cooling the air entering the gas turbine compressor which augments gas turbine power output during operation at high ambient-air temperature. The mechanical components in the cooler include an atomized-water spray-type evaporative cooler consisting of multiple manifolds and nozzles installed in the air-intake system in a location with low air velocity and sufficient residence time to enable evaporation of the water injected into the intake air before it enters the gas turbine compressor and an auxiliary system for delivering a controlled water flow to each manifold that consists of a water storage tank, water pump, piping, control valves, and a water-flow sensor for each manifold. The water injection control system starts, stops, and modulates water supplied to the evaporative cooler based on humidity or wet-bulb temperature of the ambient air, dry-bulb temperature of the ambient air, dry-bulb temperature of the air downstream of the evaporative cooler, air flow at the compressor inlet, and gas turbine operating parameters that indicate operation at or near maximum output.
A further embodiment of the water injection control system is for a divided, two-stage water-saturation and supersaturation system installed in a gas turbine air intake system for augmenting gas turbine power output during operation at high ambient-air temperature which optimizes gas turbine power output and efficiency while maintaining the water content in the intake air within limits acceptable to the gas turbine based on measured gas turbine operating parameters. The mechanical components in the divided injection system include an atomized-water spray-type saturation section with multiple manifolds and nozzles installed in a section of air-intake duct with large flow area so that the air velocity is low; an atomized-water spray-type supersaturating section with multiple manifolds and nozzles installed in the air-intake duct downstream of the saturation section or in the gas turbine""s compressor inlet hood with atomized water flowing directly into the compressor blade path; an auxiliary system for delivering a controlled flow of water to the saturating section and supersaturating section individually that consists of water storage tank, water pump, piping, control valves, and flow sensors. The water injection control system starts, stops and modulates the water supplied to the saturating section and the supersaturating section individually based on the ambient-air humidity or wet-bulb temperature, ambient-air dry-bulb temperature, compressor inlet air flow, dry-bulb temperature of the air/water mixture downstream of the saturating section, water flow to each manifold and gas turbine operating parameters which indicate high output.
Another embodiment of the water-injection control system is for a water supersaturation system installed in a gas turbine air intake system downstream of a media-type evaporative cooler for augmenting power output during operation at high ambient-air temperature which optimizes gas turbine power output and efficiency while maintaining water content in the intake air within limits acceptable to the gas turbine based on measured gas turbine operating parameters. The mechanical components of the supersaturation system includes a media-type saturation section installed in a section of air-intake duct with large flow area where velocity is low: an atomized-water spray-type supersaturating section installed in the gas turbine""s compressor inlet hood with atomized water flowing directly into the compressor blade path; an auxiliary system for delivering a controlled amount of water to the supersaturating section that consists of a water storage tank, water pump, piping, control valves, and flow sensors; a second auxiliary water system for the media-type saturating section that consists of water sump, recirculation pump, blow-down valve, potable-water makeup valve, piping etc. For the media-type evaporative cooler, the control system starts and stops the circulation pump for the media-type evaporative cooler based on the dry-bulb temperature of the ambient air, humidity or dry-bulb temperature of the ambient air, and gas turbine parameters that indicate that the gas turbine is operating at or near maximum output and modulates makeup water to the evaporative-cooler sump. For the supersaturation system the control system starts, stops, modulates and limits the water supplied to each manifold in the supersaturating section based on ambient-air dry-bulb temperature, dry-bulb temperature of water/air mixture downstream of the evaporative cooler, air flow at the compressor inlet, water flow to each manifold, and gas turbine operating parameters which indicate power output at or near maximum,